The formation of appropriate cellular or particulate layers for later optical examination is important to many fields. One of these fields is hematology, where several methods and devices have been described for obtaining clinically useful cell concentrations.
A method and apparatus for analyzing a blood or other biologic fluid sample in a quiescent state without the need for separate fluid streams passing through the blood sample during the analysis is described in U.S. Ser. Nos. 09/248,135 and 09/249,721. Although this method simplifies the analysis procedure and yields the full compliment of CBC parameters it also possesses several disadvantages. One disadvantage of the apparatus is that the concentration of cells in the examination layer is not controlled. This can lead to difficulties in optically examining cell volume and morphology. Another disadvantage of the aforementioned apparatus is that the field of cells may be too sparse in clinically relevant samples to complete scanning in a timely manner.
U.S. Pat. Nos. 5,627,041 and 5,912,134 describe an apparatus and method for cytometric measurement of cell populations using fluorescent markers. However, a disadvantage of this method is that, if the sample under test is blood, it requires addition of diluent in such quantities that white blood cells (WBCs) with depressed counts are not very numerous in the sample and may require extremely long examination times to locate them. Moreover, if the cell counts within the undiluted blood are low, clinically relevant cell populations present in the sample may not be detected in the diluted sample. For example, patients who undergo chemotherapy regimens may have depressed white cell counts in the range of 1000 cells/μL and less. Cytometric examinations are typically searching for a sub-population of these cells, further reducing the likelihood of locating them.
U.S. Pat. No. 4,790,640 discloses a wedge shaped device for trapping rigid particles, such as sickle cells in blood. However, a disadvantage of the device is that the selection of cell sizes is accomplished by thickness of the chamber alone, which can exhibit substantial manufacturing variation over the examination area, causing a corresponding loss of ability to separate by size.
Consequently, it would be desirable to have a method and apparatus for obtaining the desired cellular concentration in a blood or other biologic sample which can mitigate the effect of a separate dilution step and addition of diluting fluids.